Video switching systems such as those used in the closed circuit television (“CCTV”) industry often operate with non-synchronized cameras, i.e. the blanking intervals on the cameras are not synchronized to occur at the same time or synchronized with the switch. When different cameras are selected, the display monitor often loses vertical lock, and the image “rolls” badly until it re-locks. This situation can be improved in the cameras when the command to switch is synchronized with the input alternating current (“AC”) power line frequency. As such, it is desirable to switch the video signal during the vertical blanking interval to prevent roll and start the new picture frame at the beginning of the next raster scan.
Prior art devices have been used to synchronize video switches to the 60 Hertz power input using optical couplers, capacitors, or zero-crossing detectors such as triacs. However, these prior art devices necessitated one of two undesirable solutions. First, the 120 volt AC line signal must be brought onto a circuit board or coupled to a circuit to perform the synchronization function. Second, a common way to lock to the power line is to detect zero-crossings at the low voltage secondary of the input power transformer by using a comparator. However, with contemporary direct, off-switching power supplies, such a transformer is not used. As such, this solution is impractical. Also, if an attempt is made to sample the raw input line, the detector must operate at high voltages, provide line isolation, and meet all certifications such as Underwriters Laboratories (“UL”), Federal Communications Commission (“FCC”) and other related certifications. This is expensive and difficult.
Systems for synchronizing a plurality of cameras using a master source located at a central station are known. However, the device at the central station is typically a highly-complicated device which distributes both horizontal and vertical synchronization signals. These systems do not, however, provide a synchronization signal to a central video switch which is used to switch between cameras during their vertical blanking interval.
It is therefore desirable to be able to have a system and method which avoids the need to sample the raw input line and cause the detector to operate at high voltages. It is further desirable to have a system and method for providing a synchronization signal based on the AC power line signal which does not require certifications and which allows the use of contemporary direct off-line switching power supplies without the need for a separate transformer.